OK, have more time than earlier now to answer this question!
Personally, I think that the issue with talking on a mobile goes beyond the actual physical you-need-to-use-a-hand-to-hold-it risk.
When you talk on the phone, you are concentrating about the conversation you're having. This is also the case when you are driving and chatting to passengers in the car, but there's one crucial difference - if they're in the car, they can see when you're joining a motorway, overtaking, navigating a roundabout in heavy traffic, etc. On the phone, someone can continue asking you complicated questions/ranting/telling you funny things, etc and not know what you're doing in the car - therefore they don't modify their conversation at all to take account of this and this can be very distracting as you're trying to drive.
I think that even hands-free phones are very dangerous and I won't use a mobile at all in any way in the car. Also, if I ring someone and they're driving, I ask when they're likely to be at their destination and call them back. Imagine the horror of hearing crumpling metal and screams when you're on the phone to someone who's driving...